In blasting operations, a variety of electric and non-electric devices may be used to amplify and transmit a blast initiation signal including detonators, ignitors, delay detonators, initiators and the like. As used herein, the term "detonator" is intended to generally refer to the variety of devices which amplify and transmit an initiating signal.
Detonators are generally comprised of an elongated tubular housing closed at one end and open at the other end, and contain one or more pyrotechnic or explosive charges positioned within the housing adjacent the closed end.
The output of the detonator is proportional to the density and quantity of the explosives as well as their chemical composition and, therefore, a small diameter press pin is used to compress the explosives within the housing. To achieve the desired explosive density, pressing forces as high as 300 pounds (6,000 psi on a 0.254 inch OD pin) and higher may be used.
It has been found that many explosives are more sensitive to initiation by impact or friction if compaction energy is applied to the surface of the explosive using a hard surface. The degree of sensitization is a function of the hardness, i.e., increasing hardness causes increased sensitivity. Because of the high pressures required to achieve the desired detonator explosive density, pressing of explosive within the housing is usually accomplished using a press pin, typically steel in direct contact with the explosive, thereby causing sensitization of the explosive and increasing the hazard associated with pressing the explosive.
The combination of the hardness of the press pin, alignment of the tooling, the breaking of the explosive into smaller particles during compression, and the explosive being under high compression, provide conditions which, unless very carefully controlled, can lead to inadvertent and unwanted activation of the detonator.
Another potential mechanism for inadvertently initiating the detonator during compression of the explosive is displacement of small particles of explosive into the annular space between the pin and the housing as consolidation of the explosive occurs. As the press pin is in motion while being inserted into the housing, while pressing the explosive material or while being withdrawn from the housing, particles of explosive material trapped between the housing and the press pin surfaces are subjected to a substantial amount of friction which could lead to unwanted detonation.
After compression of the explosive within the housing, the detonator is usually shipped between manufacturing facilities as well as to the use site, and during such shipment, the detonators are subjected to vibration, shaking and shock which tends to loosen the compressed explosive material. The loose particles of explosive are typically very sensitive to initiation by friction and static electricity and therefore present a safety hazard during transportation and subsequent handling of the detonator at the new location.